Thermal batteries are thermal activated, primary reserve, hermetically sealed power sources, generally consisting of series or series-parallel arrays of cells. Each cell is comprised of an anode, an electrolyte-separator that is solid and no-conductive at room temperatures, a cathode and pyrotechnical means. The cell is activated by providing sufficient heat to melt the electrolyte.
A variety of electrochemical systems are known for use in thermal cells. The electrolytes are generally mixtures of alkali metal halides, most commonly eutectic mixtures of LiCl—KCl (melting at about 352° C.) and LiCl—LiF—LiBr (melting at about 440° C.) although other fusible salt mixtures have been used, such as alkali metal thiocyanates. etc. Common cathode materials, among others, are iron pyrite, cobalt sulfide, calcium chromate, copper chloride and copper oxide.
Typically, pure lithium metal is used as an anode, however, due to its high reactivity, some prominent disadvantages exist, among them is the formation of lithium nitride. This compound serves as a catalyst for its continuous formation, in particular, during nitrogen leakage into the battery during aging period, resulted in a gradual conversion of the metallic lithium anode into said nitride. This phenomenon has been found to seriously degrade the life time of a thermal battery. This problem of lithium nitride formation was never totally solved and elimination of this compound, remained a source of concern throughout the years.
Due to lithium's high reactivity the anode preparation requires very difficult maintenance of high purity argon gas to prevent lithium nitride formation. Even with expensive appropriate equipment, the lithium-based material, once it is formed, becomes tarnished after cooling. The mat gray film formed as a result of the precipitation of lithium nitride and other impurities (R. Szwarc and S Dallek, “The Li(B) Ingot Preparation Scale-Up Study—Final Report”, GEPP-TM-645, General Electric Company, 1982).
There are two reasons for lithium nitride formation during lithium melting. One is that a “nitrogen free” atmosphere generally comprises about 1 ppm of nitrogen. This concentration, although very small, cannot be considered as a “free” or “zero” nitrogen atmosphere. Said small amount of nitrogen is enough for reacting with lithium to produce an impurities amount of lithium nitride in the lithium-based anode material. Further, nitride impurities are always present in the lithium raw material. Such impurities, even in a very small quantity, are inevitable due to the existence of an eutectic composition between lithium and lithium nitride.
The eutectic composition comprises 0.068% mol nitrogen {P. Hubberstey, R. J. Pulham and A. E. Thunder, “Depression of the freezing point of lithium by nitrogen and by hydrogen”, J. Chem. Soc. Faraday Trans., 1 [72] 431-435 (1976)} and always causes lithium raw material to contain lithium nitride impurity.
U.S. Pat. No. 3,930,888 discloses active anode metals, including alkali metals, alkaline earth metals or alloys thereof that melt below the cell operating temperature, or, for most purposes, below about 400° C., preferably lithium or an alloy of lithium and calcium. Use of liquid lithium anode in thermal batteries provides a number of advantages among them are its capability of providing high voltage, power density and energy density.
The active anode metal is carried by a foraminous metal substrate that is wet by the molten anode metal and is substantially inert to electrochemical or other reaction in the particular cell system used. The substrate is filled with active anode metal, most suitably by dipping the substrate in molten anode metal, withdrawing the substrate and then cooling it below the melting point of the anode metal; when the anode metal is melted on activation of the cell it will then wet and fill the substrate.
The anode housing comprises an impervious inert metal portion and a porous refractory fibrous portion. The metal portion is in electrical contact with the anode metal and may be of any solid metal substantially inert to the other cell components with which it may contact, preferably nickel, stainless steel or iron. The porous portion (dry asbestos fibers and/or any insoluble, inorganic, non-metallic fibers of high melting point that is infusible during operation of the cell, such as refractory or ceramic fibers, either acidic, basic or amphoteric, may be used) of the housing is in tight engagement with the entire periphery of the metal portion of the housing in order to prevent leakage of the molten anode metal along the metal housing surface to the exterior of the housing, that would cause shorting or other premature failure.
A major disadvantage of this anode lies in the reactive nature of lithium and its low melting point (about 180° C.) which may result in a leakage of the molten metal, and consequently may cause short circuits and premature failure in such batteries.
U.S. Pat. No. 4,221,849 relates to an anode material comprising a pyrometallurgically combined iron-lithium anode for use in lithium anode thermal batteries. The ratio of lithium to iron is about 15% to 35%. In these ratios, the iron particles are held together by the surface tension of the lithium rather than being alloyed thereto. The lithium is heated to about 500-600° F. and the iron added in particulate form while stirring the molten mixture. The iron-lithium anode disk is positioned in a metal cup by means of an inert insulator or separator ring preferably, made of Fiberbrax®. The electrolyte, normally in the form of a wafer, is positioned adjacent to the separator in the cup.
It has been found that an activation of thermal batteries assembled with said iron-lithium anodes formed a noise of a few seconds duration, with peak-to-peak values of greater than 0.5 volts (between 3 and 15 KHz). The noise which is greatly exaggerated in batteries operating in cold conditions as compared to those operating in warm or hot conditions, has found to seriously degrade the final activation rate of these batteries.
U.S. Pat. No. 4,675,257 which uses the same anode of the U.S. Pat. No. 4,221,849 comprises a metal cup and a metal screen interposed between the metal cup and anode composite material. The positioning of a metal screen between the metal cup and the anode composite material together with removal of the fiberfrax separator resulted in the reduction and elimination of activation noise and improved the electrical characteristics of the battery.
The above three US patents have the major disadvantage of undesired development of lithium nitride as discussed hereinabove.
U.S. Pat. No. 4,781,756 teaches a process for the removal of lithium nitride from high purity lithium metal by adding a stochiometric amount of aluminum to liquid lithium metal containing lithium nitride (at a temperature between the melting point of lithium and 300° C.) to react with the lithium nitride, in an inert, nitrogen-free, atmosphere to form aluminum nitride, and subsequently separating the aluminum nitride from the liquid lithium metal by settling and filtering the mixture using a 0.5 μm filter.
U.S. Pat. No. 5,019,158 deals with a process for separating calcium and nitrogen from lithium, in which alumina is added to a molten lithium and reacts to produce aluminum and lithium oxide. The aluminum reacts with the nitrogen in the lithium to produce insoluble aluminum nitride, while the lithium oxide reacts with the calcium present to produce insoluble calcium oxide and lithium. The insoluble calcium oxide and aluminum nitride may then be separated from the molten lithium (for example, by filtration). This operation is preferably takes place at temperatures between 200° C. and 250° C.
The latest two discussed patents (U.S. Pat. Nos. 4,781,756 and 5,019,158), suffer from the disadvantage of having a filtration step which is very difficult to perform whenever high viscous mixtures exist. More specifically, they include a filtration step to remove the insoluble oxides and nitrides from the molten lithium metal. As a result, they cannot been used in the process for production of iron-lithium anodes, due to the fact that the pyrometallurgically combinations of iron and lithium (15-35wt % Li) form highly viscous mixtures that are not filterable.
U.S. Pat. No. 4,158,720 discloses a lithium-aluminum-iron alloy for use in a negative electrode within a secondary electrochemical cell. It was found that such electrodes exhibit increased electrode potential over that of electrodes containing only lithium-aluminum alloys. The anode composition comprises about 5-50 atom percent lithium and about 50-95 atom percent alloy of aluminum and iron. The aluminum and iron alloy includes about 20-35 atom percent iron.
The aluminum-iron alloy (Fe2Al5), when saturated with lithium, provides an increased lithium activity and consequently increased electrode voltage over that of a comparable lithium-aluminum alloy. The electrode material is prepared by first providing an alloy of aluminum and iron and then electrochemically depositing lithium into a porous mass containing that alloy.
There is a major drawback to this US patent that should be discussed. The anode material consists of an alloyed lithium compound (Li—Al—Fe alloy), and consequently suffers from lithium not being present in an active metal form. The absence of lithium in an active metal form resulted in a substantial reduction in the anode's potential.